Supertest/mocha Error: global leak detected: path

So I'm trying to write tests for my REST API (built on top of Express and Mongoose), but I have run into some trouble.

I have followed a lot of examples and tutorials, which suggest that my solution below should work, but it isn't - I am getting a Error: global leak detected: path

It seems that the line that is causing it is .post( '/api/invoices' ) - but I cannot figure out why.

var app = require("../app").app,
    request = require("supertest");

describe("Invoice API", function() {

    it( "GET /api/invoices should return 200", function (done) {
        request(app)
            .get( '/api/invoices' )
            .expect( 200, done );
    });

    it( "GET /api/invoices/_wrong_id should return 500", function (done) {
        request(app)
            .get( '/api/invoices/_wrong_id' )
            .expect( 500, done );
    });

    it( "POST /api/invoices should return 200", function (done) {
        request(app)
            .post( '/api/invoices' )
            .set( 'Content-Type', 'application/json' )
            .send( { number: "200" } )
            .expect( 200, done );
    });

});